the build article by Steve Griffiths and Mike Briggs - 16/1/12
Some time ago, Mike and I identified the need for an aerobatic model that would cope well with the windy conditions which seem to predominate in our neck of the woods, yet would be equally suitable for those infrequent days when relative calm is restored to our favourite slope. Eventually we came up with the Typhoon, a model with an RG15 wing section that penetrates well, gives a good turn of speed yet, with a small amount of flap, will soar in quite moderate lift.
This is a nicely versatile aeroplane, with space in the fuselage for ballast that, similarly, enables it to cope with a wide range of wind speeds. Oh, and at just under 60” span, it qualifies as a slope racer too. Typhoon’s fairly clean design is virtually silent in flight, with none of the reported ‘howl’ that some say is associated with the RG15 section.
We both built a prototype, each fitted with Ripmax SD200 servos on elevator and rudder, and metal-geared Hitec HS80s on flaperons. Larger servos could be used in the fuselage with some adjustment to snake runs, whilst bulged fairings may be needed to cover thicker wing servos.
As you’ll see from our plan, the design incorporates veneered blue foam flying surfaces, with the whole covered in glass cloth. Incidentally, please don’t let that put you off - read the section entitled ‘Easy Glassing’ and trust me, you’ll not only find it much easier than you thought, you may even enjoy the experience!
If you’re happy with the glassing aspects, it’s quite an easy construction project for builders with moderate experience, and can have either a conventional or V-tail configuration. Sheet tail-feathers could be fitted, but we haven’t done that, so it’s up to you to experiment if you want to go that route. Ailerons and elevator are top hinged, while the rudder is centre-hinged. One prototype has only Robart ‘point’ hinges, the other has tape hinges on ailerons and elevator, with leaf hinges on rudder. Okay, let’s look at some finer points of the construction.
If you plan to cover the model in anything other than glass cloth (up to 1oz. / sq. yd.), then please feel free. You will inevitably lose some strength, but may be able to replace it by fitting suitable wing spars. If you go the glass route, complete any surface decoration, then laminate the cloth over the top.
Those who aren’t happy working with epoxy or polyester resins could try using acrylic varnish, which is a much easier medium to apply. Available from most DIY stores, it’s virtually odourless, certainly less toxic than epoxy, needs no hardener, sets more quickly and, before it dries, can be cleaned off tools and brushes with water. It is also stickier than epoxy, so lightweight glass cloth can be teased round small-radius corners / leading edges, and will stay there.
Several coats will fill the weave without adding too much weight, each being applied as soon as the previous one is dry. When fully hardened, it may be rubbed down with wet-and-dry sandpaper (used wet) and finished off with rubbing compound, to give a very smooth surface.
Prototype No.1 was built largely using pre-coloured materials, with other parts coloured prior to glassing - no further decoration was applied. Eventually, the T-cut treatment was given, and I have to say, the result is extremely glossy and very pleasing.
OVER TO YOU
No blow-by-blow building account here; I’m sure you know how to assemble a wing, so I’ll simply provide you with a few explanatory notes, first of which deals with the ailerons.
The latter are marked, but not cut out until after the wing panels have been capped (leading edges etc.) and joined - only then are the facing strips applied. The aileron cutting area should be highlighted with a series of lines on the upper and lower panel surfaces, to show the root and tip extremities, along with the apex (on the upper surface) and base lines (lower surface) of the triangular section that must be removed to facilitate the hinging process (Fig. 1).
When sanding the root sections, angle them so as to produce zero dihedral across the upper surface. During fabrication of the laminated tip blocks, it’s worth noting that the ply layer should be aligned accurately on the chord line to provide a hard, thin trailing edge.
In order to accurately cut the ailerons, slice through the undersurface veneer, using your previous marks as a guide. Remove the veneer from between the cuts. From the top, slice right through the wing with a cut along the hinge line, then free the surface with a further chord-wise incision at each end. Trim and sand the foam back to the veneer, shorten the ends for clearance, add hard balsa blocks for hinges and horns, then cover the raw faces with glass-cloth, sanding the edges flush when dry. Finally, add the aileron hinges as close to the upper surface as possible.
ANYONE FOR ‘T’?
Prior to building the tail it’s worth noting that, on the conventional set-up, the elevator root sections are cut away to clear the rudder, and chamfered to clear each other on the V-tail.
When building the conventional tail, follow the same sequence of operations as that for the wing, but omitting the joint braces. In order to achieve a straight hinge line, the upper surface should have zero dihedral. For a suitably flush-fitting rudder, a good dodge is to glue the fin-post centrally onto the rear edge of the fin, then tack-glue the rudder to it before shaping to section. Finally, cover with glass cloth, cut the rudder free, then chamfer and hinge.
For the V-tail, build in the same way as the conventional tail, but complete all work before joining. When set, make a saw cut across the underside and epoxy-glue the 1.5mm PCB joint brace. Sand off any excess when cured.
References to fillets on formers imply the use of 1/4” triangular stock, sanded to conform to the joint. Note that the 3/16” sq. longerons are scarf jointed at the back of the doublers. Fuselage sides are joined using slow-setting (minimum one-hour) epoxy, which fixes F2, F3 and the wing bolt-plates in position. Remember, the bolt-plates cannot be inserted after the sides are joined, so please don’t forget them. Those fillets mentioned earlier can now be applied to both edges of F2 and 3, front and back.
If building the conventional tail, taper the inside faces of the longerons to ensure that the rear end is 1/4” thick when the sides are pulled together at the back. Glue the tail end together; fill between the longerons, top and bottom, with 3/16” sheet, as far forward as the leading edge of the tailplane.
For the V-tail, glue the rear end together. Fill between the bottom longerons with 3/16” sheet as far forwards as the tailplane leading edge, leaving the rear ends of the fuselage sides 1/2” - 5/8” apart to accommodate the torque rods and linkages. Fill between the top longerons, an inch or so forward of the tailplane l.e., using 3/16”.
Snake tubes must be prevented from flexing by supports, positioned at no more than 4” intervals. Unless tall servos are installed up front, snake outers should pass beneath the wing bolt plates. While we’re on the subject of conduits, the aerial tube should not project forward of the rear bolt plate - if it does, fitting ballast will become difficult.
When adding decking to the fuselage, you’ll notice that the joint between the underside pieces is supported by F3, and that the rear upper section is rebated at its front end to fit between the ply sides.
Before putting some more sinuous contours on the fuselage, make sure that the wing is square, and drill through said flying surface before bolting the plates together to ensure accurate alignment. Check the tailplane fit, and its accurate alignment with the wing. Notice that, in order to provide good contact for gluing, the inside edges of the tailplane seat are chamfered; only a little for the conventional tail, more for the V-tail. For the conventional tail, trim the front of the fin to fit closely to the fuselage top, and mark round it to show the limit for shaping - alternatively, it can be notched into the top. When the tailplane and fin are eventually fixed in place, reinforcing rods are employed to pin them securely to the fuselage.
Remove the wing and tailplane, then finish the fuselage by shaping it to a nicely rounded section, leaving the area marked for the fin flat to give a good glue joint. Cover the fuselage with glass cloth, before fixing the tailplane in place. Finally, laminate, shape and fit the tail-skid.
When carrying out final assembly of the various components, note that Robart ‘hinge points’ should be glued into blue foam using epoxy, although PVA is acceptable for fixing into balsa. Fit the servos, verify full and free control surface movements, and balance in the middle of the range shown, i.e. at 3.6” behind the leading edge. This is marginally ahead of 30% MAC (Mean Aerodynamic Chord).
Choose a day with good lift for the first flight, preferably not too turbulent. It’s pointless suggesting you wait for a wind of a given speed, since what that actually equates to in terms of lift is obviously dependent on your slope. Having said all that, it’s unlikely you’ll get the best out of the model in winds of less than 10mph unless your slope is an absolute stormer.
For early flights, set rates to limit the control surface movements, using about 1/2 or 2/3 of the stated values. At speed, it’s pretty quick on the roll, especially with full movement. Launch with a good push straight out, and let it gain momentum - you may feel more secure on the first flight with a little up-trim, especially if you’ve balanced the model slightly forwards for safety.
Gain some height, check stall behaviour, and try the brakes. A few feet from the ground on your final approach is no time to find that you have a sudden nose-down pitch when you deploy the latter - discover the truth with a good helping of insurance beneath you.
When you’re satisfied with the model’s general handling, try some manoeuvres and find out what it’s capable of by turning the rates off. If you’re feeling frisky, increase aileron deflection to 1” up and 3/4” down; then, you’ll really see it roll.
Typhoon performs better when flown fast, but is no slouch even at moderate speeds. In truth it’s one of those deceptive aircraft that, when apparently flying quite slowly, will loop with ease. Pitching manoeuvres are aided by coupling flap to elevator (‘down’ flap with ‘up’ elevator and vice-versa), setting the mix to give about 1/8” flap with full elevator movement. More speed can be attained by reflexing the ailerons 1/16” - 3/32”; experiment to find out exactly how much suits you, and have a real ball!
The balance point can be moved rearwards to improve pitch response - indeed, she retains adequate stability balanced as far as 4.0" behind the leading edge (38% MAC). beyond this, it hasn't been tested!
Designed by: Steve Griffiths & Mike Briggs
Wing area: 437sq. in.
All-up weight: 42 oz. without ballast.
Wing loading: 14 oz. per sq. ft.
Wing section: Girnsberger RG15; 9in. root chord, 6in. tip chord.
Tail and fin section: SD8020
Control functions: Flaperon, elevator and rudder
Incidences: 1° (wing) 0° (tail) relative to datum.
Centre of Gravity: 3.2 - 4.0” behind leading edge at root
Control surface travel: Aileron - 3/4” up; 1/2” down
Lift flap - 1/16 - 3/32” down
Elevator-coupled flap - 1/8” each way with full elevator in opposite sense
Brake flap - 1” up, coupled with 3/32” up elevator
Conventional tail: Elevator - 3/8”each way; rudder - 11/2” each way
V-tail: Elevator - 1/4” up, 3/8” down; rudder - 1/2” each way